Atmospheric fallout of nuclear weapon test that most occurred in the fifties and in the sixties involved activation products isotopes such as 238Pu (half-life = 87.7 y), 239Pu
(half-life = 24 131 y), 241Pu (half-life = 6 537 y), 241Pu (half-life = 14.4 y) as well as radiogenic 241Am (half-life = 432 y) daughter product of 241Pu. Thus atmospheric source terme was well characterized from activity ratios. For example, 241Am/239Pu+241Pu ratios measured in the air and in rain has allowed to estimate residence time of radioactive dust in the atmosphere, after each nuclear shots.
As a consequence soils are nowadays the largest environmental reservoir of transuranic elements. In soit, 24'Am originates from atmospheric fallout and from in situ decaying 241Pu. Thus from a theoretical point of view soil Am/Pu ratios are time dependent and yield the age of Pu deposition onto soil. Therefore soil Am/Pu ratio provides unique opportunity to trace the behaviour of transuranic elements in soil, to check if 241Am/239+240Pu and 241Pu/239+240Pu ratios are time dependant or if Am/Pu chemical fractionation occurred.
We studied soils from high altitude catchments (French Alps and Vosges mountains) where strong radioactive fallout are due to large precipitation amount. Soils exhibit two distinct Am/Pu ratios :
- ratios close to the theoretical signature expected from atmospheric reservoir characterize "reference soils"
- anomalous soils exhibit lower Am/Pu ratios than expected. Despite such low daughter activity, 241Pu/239+240Pu ratios of those soils are consistent with primary atmospheric fallout ratio. In this case Am/Pu ratios trace Am and/or Pu migration in soils. Assuming Pu is immobile with respect to Am, we calculated that anomalous ratios are consistent with migration of 25-40% of Am inventory. Transuranic elements migrations will be further characterized from soil/water interface chemical properties.